To confirm the application prospect of the obtained NiFe
2O
4‒δ, an ASC composed of a NiFe
2O
4‒δ positive electrode and an active carbon negative electrode was fabricated. Results of electrochemical performances of the fabricated ASC are shown in Fig.5. Fig.5(a) displays CV curves of the obtained ASC at different scan rates. As we can see, a high 0–1.5 voltage window is achieved in this device. This is powerful evidence that the obtained NiFe
2O
4‒δ has good application potentials. To further judge the energy storage properties of the assembled device, GCD curves were obtained through chronopotentiometry method, showing in Fig.5(b). Calculated by Eq. (1), specific capacitances of the device at current densities of 0.5, 1, 2, 5 and 10 A·g
‒1 are attained to be 56.7, 45.3, 36.0, 30.0 and 26.0 F·g
‒1. The slight polarization in CV curve as well as voltage drop in GCD curve are caused by the intercalation and de-intercalation of OH ions in a solid phase structure [
45,
46]. Fig.5(c) shows Ragone plots of the obtained NiFe
2O
4‒δ//AC device and other researchers’ works [
47–
50]. It can be seen that at a power density of 375 W·kg
‒1, the energy density of the obtained NiFe
2O
4‒δ//AC device can reach 17.7 Wh·kg
‒1, demonstrating good application performance. In addition, Fig.5(d) displays the cycling stability and coulombic efficiency at 3 A·g
‒1. As seen, the device can maintain 98.7% capacitance after 5000 cycles with a high coulombic efficiency of 99.4%, preforming excellent reversibility and cyclic stability. Besides, Fig.5(e) and 5(f) show the SEM images of the positive electrode before and after cycling. It can be seen that NiFe
2O
4‒δ nanosheets and carbon particles are evenly mixed. After 5000 cycles, the morphologies of NiFe
2O
4‒δ nanosheets hardly changed, indicating the excellent cyclic stability. All these attractive results indicate that NiFe
2O
4‒δ//AC ASC device has good electrochemical performance and enormous application potential.